Usually they spawn on the night when both moons are tiny slivers in the sky. This happened a day or so ago, so it will be about 3-4 real days before it happens again.

There have been other people whom have said they spawned on other nights as well, but they are almost guaranteed on the two slit moon. I have found them 10 times in the spawn locations myself just by going by the phase of the moon.

You may have to camp out the three moon cycles of the day that happens to catch them in one camped out spot. An alarm clock in handy for that in case you fall asleep.

Here is an interesting page for you to read that should help.

http://forums.elderscrollsonline.com/en/discussion/64587/vampire-guide-everything-you-need-to-know-zos-please-read-the-cured-section/p1

http://elderscrollsonline.info/guides/vampire